The present invention relates to digital coders/decoders, and more particularly to a frame format for a video codec for transmission of video data over a digital transmission network at a DS3 level while providing for incorporation of information at a DS1 level.
A digital transmission network is made up of sources of digital signals, including channel banks and multiplexes, and transmission facilities. These operate at different bit rates. At any one level in the digital hierarchy there may be several signal sources with unique bit stream formats, but they must have certain common characteristics to permit interconnection with transmission facilities at that level, and multiplexes connecting to a higher level. The designation DSN refers to those common features of the digital signal at the Nth level in the network as defined by Technical Advisory No. 34 by American Telephone and Telegraph Company, Network Planning and Design Department, Basking Ridge, N.J. The present levels in the Bell System digital network include DS1 (1.544 Mb/s) and DS3 (44.736 Mb/s).
For the transmission of video information over the digital transmission network the DS3 level is used. The DS3 format as shown in FIG. 1 consists of a master frame having seven subframes, each subframe having eight 85-bit words as shown in FIG. 1, and must be used by all DS3 sources. Each 85-bit word has 84 information bits and one control bit. A frame alignment signal (F1F0F0F1) is used to identify all control bits, and a multiframe alignment signal (M0M1M0) is used to locate all seven subframes. At the beginning of the first and second subframes are X-bits which must be identical in any one master frame, i.e., either 11 or 00. The X-bits may be used by a DS3 source for asynchronous low speed signaling. The source may not change the state of the X-bits more than once every second. At the beginning of the third and fourth subframes is a P-bit which contains parity information. DS3 sources must count parity over the information bits (8.times.84.times.7=4704) following the first X-bit in a master frame and insert the resulting parity information in the P-bit positions of the following frame. PP=11 if the digital sum of all information bits is 1 and PP=0 if the digital sum of all information bits is 0. The remaining control bits, called CB, are used by DS3 sources.
For NTSC video the subcarrier frequency is 3.579545 MHz. If the video is sampled at 2.5 times the subcarrier frequency and digitized to four bits, the video bit rate is 35.795454 Mb/s. For transmission over the digital transmission network using the DS3 format it is necessary to place the video bit data at 35.795454 Mb/s into the DS3 format at 44.736 Mb/s. Prior television codecs use the DS3 clock as the sample rate. Since the DS3 rate is asynchronous with the video rate, the result is a certain amount of chroma noise and sampling related chroma artifacts.
What is desired is a video codec which can encode and decode video data into a DS3 frame format for transmission over the digital transmission network to provide video data with reduced chroma noise and sampling related chroma artifacts while permitting more data space for additional data within the DS3 frame, such as DS1 and/or other ancillary data.